See how extreme it would have to be for choosing between two candidates to be forbidden. Short of that, "lesser of evil" is a misnomer escaping tobacco stained lips of bullshitters who can't even spell Catholic.
[Double-effect] allows you to choose a candidate who has some good and some evil positions ... as long as the double-effect criteria are met.
And the example I gave would be about the only scenario in which you would not have a double-effect alternative for whom you could vote.
There are lots of double-effect scenarios which would fail the test.
Let's say that there's a Pro Abortion candidate whose economic policies are great. I vote for that person's economic policies with the unintended / unwilled secondary effect that the candidate is Pro Abortion. This fails the double-effect test that the good effect must outweigh the bad effect (economic policy does not outweigh life issues). Some "Catholics" attempted to justify their vote for Obama on these grounds.
Double-effect does NOT let you select between two evil candidates. You cannot make one evil choice in order to prevent a more evil choice (that's actually lesser evil, not double effect). See, the act itself that occasions the unintended evil effect must in itself be either good or indifferent, and cannot be bad. It also violates a double-effect principle because the good effect (blocking the worse candidate) happens as a result of the bad effect (choosing a bad, albeit less bad, candidate).
Double-effect does clearly apply to Trump, however, so IMO any Catholic can vote for him.
Now, as Matthew pointed out, we are usually not under strict obligation to take a double-effect action. So, for instance, I could fail to kill someone else to protect my own life, thereby losing my life, without committing grave sin. In fact, it would be considered heroic virtue if one does it out of love for the assailant. So, for instance, women sometimes allow their own lives to be lost in childbirth rather than taking some double-effect remedy to save their own lives. That too would be heroic virtue.
On the contrary, however, let's say that I see a THIRD PARTY being violently attacked and I have a gun. In that case, I AM under obligation to defend the life of the victim even through lethal force.
In conclude that, in this case, that of a Pro Life (at least ostensibly) candidate vs. a Pro Abortion candidate, provided that the Pro Life candidate can be voted for at least by way of double-effect principles, a positive obligation exists to take that action. In other words, I liken this to the situation of seeing an innocent victim being assaulted and having to take lethal action in order to save that person's life.
So, based on Catholic principles, I don't see how a Catholic is not actually obligated to vote for Trump.